The present invention relates to a magnetic disk and a magnetic disk device for reading and writing data, and specifically to a magnetic disk with a special format providing reserve servo sectors and reserve tracks, and to a magnetic disk device which uses this magnetic disk for improved track access performance and data protection functions.
Magnetic disk devices have become widely used in primary data storage systems due to their ability to store large volumes of data and to randomly access this data at high speed. Furthermore, unlike tape drive devices, high speed data access can be implemented because the data is recorded to plural concentric data recording tracks on the surface of the disk platter.
Many common hard disk devices today comprise plural circular hard disk platters, produced by forming a magnetic film on an aluminum or glass substrate, assembled on a common spindle in a disk assembly, a head carriage assembly of plural magnetic heads providing one magnetic head for each surface of the disk platters, and a positioning device which drives the head carriage radially to the disk platters for track access. In this hard disk device, one side of each of the stacked disks is not used for data storage, but instead is used to store the servo signal pattern required to position the head carriage. The magnetic head is thus positioned at the selected track by reading this servo signal pattern. This access control method is known as the dedicated servo method. Because the data signal recording surface and the servo signal recording surface are physically separated in this type of hard disk device, the data recording tracks and data sectors of the data signal surface can be formatted in any way. In addition, the head can be positioned at high speed to any desired track regardless of the disk format. This type of device is described in U.S. Pat. No. 3,731,177.
However, it is difficult to implement this dedicated servo method in a hard disk device having only one or two disk platters rather than plural stacked disks because it is inefficient to use one whole side for storing the servo signal pattern. In hard disk devices in which only one disk platter is used, this dedicated servo design has been replaced by a embedded servo method in which the servo signal pattern is inserted between data sectors (an extremely narrow servo sector is inserted). This method is also known as the sector servo or embedded servo method. Basically this is a hybrid design in which the servo signal patterns required for head positioning are dispersed across the data signal surface.
Flexible magnetic disk devices using a polyethylene terephthalate or similar flexible medium coated with a magnetic film as the magnetic disk are the most commonly used type of removable magnetic disk devices. These so-called "floppy" disks present a problem similar to the disk access control problem of a hard disk device with few disk platters. There have been recent attempts to develop large capacity floppy disks with a storage capacity of 10 MB or more. Such disks have a very high track density and small track pitch, thus requiring a high precision servo positioning mechanism. As a result, this sector servo design is also used on these high capacity floppy disks. An example of such a device is described in Japanese patent laid-open publication No. S63-173282.
In this sector servo method the servo signal pattern can only be detected at a constant sampling time (the scatter time) no matter where the head is located because the servo sectors are scattered between the data sectors. Therefore, the controller loses track of which track the head is crossing during track access. A method by which an absolute track number is written to the servo signal pattern as described in Japanese patent laid-open publication No. S54-45113 attempts to resolve this problem. However, with this method the servo signal pattern length increases, the servo sector length thus becomes too long and the data storage area is reduced, and the servo sector is not very resistant to defects in the magnetic film. In addition, the decoder required to interpret the absolute track number is complex.
Another sector servo method whereby all of the tracks are allocated to plural small track groups, a signal pattern or number which identifies each track in the small track group is written to the servo signal pattern, and these small track groups are repeated for the disk is also known, as described in U.S. Pat. No. 4,032,984. With this method the length of the servo sectors is reduced and the data storage area is increased, and a complicated decoder is not required.
In a sector servo-type disk device using disks in which the servo sectors have servo signal patterns by which the data recording tracks are divided into plural small track groups repeated at a regular cycle, it is possible to detect from the servo signal pattern where in the small track group the magnetic head passed during track access, but it is not possible to detect which small track group was passed. However, if the time between servo sectors, i.e., the sampling time, is reduced, the maximum track access speed is appropriately limited, and the number of tracks in each small track group is increased, and thus it is not difficult to estimate which small track group is passed at each sampling time during track access, if the track position when track access is started is known.
However, there is the possibility that a seek error causing the magnetic head to be positioned at the track of the same number or type in an adjacent small track group will occur when accessing a track in any small track group. This error will not be known until the magnetic head is positioned, the track number in the identification region at the beginning of the data sector is read, and this is referenced, but when the error is confirmed as a result of this operation a re-access is executed. However, if a seek error occurs or magnetic head overshoot occurs when accessing a small track group at the extreme outside or inside circumference of the data recording tracks, there is no way to confirm the magnetic head location because there are no tracks beyond the area being accessed. Normal servo operation cannot be applied in these areas. The normal access control sequence cannot be used to move the head from an unknown disk location, and a separate special re-access control sequence is necessary. This control sequence requires preparation of a separate microprocessor program, and results in an unavoidable and extreme drop in access speed.
This problem is particularly severe with removable floppy disk drives, and especially with high capacity floppy disk drives. Temperature and humidity can cause the flexible medium in the floppy disk to expand and contract, and there is an avoidable degree of track eccentricity resulting from loading and unloading the disk, causing the track positions to vary widely with disk speed. Because the track density is high and the track pitch is narrow, the possibility of the head being positioned in an adjacent small track group is high with large capacity floppy disks. In addition, there is also a high possibility that the magnetic head will temporarily overlap the adjacent small track group due to overshoot and track eccentricity during track access.
Furthermore, protecting the data written to the data recording tracks has become an important consideration for magnetic disks, and particularly for removable disks, but the standard floppy disk formats conventionally used do not provide for any special data protection. Therefore, irregular methods have been unavoidably used to protect the data by the disk itself. Normally, an irregular format which cannot be read by the normal disk operating system for file control is used to inhibit easy file copying and thereby protect data, but these irregular methods are not guaranteed as a way to use the disks.
In addition, the security of data stored to conventional floppy disks has conventionally been a function of the system software, and has not been protected by the disk itself.